In the great journey towards Chinese-style modernization, environmental governance modernization is the carrier of governance response and verification of Chinese-style modernization. This Chinese-style modernization represented by “environmental governance modernization” is a re-modernization of modernity itself, and it is necessary to re-understand and grasp the “modern” attributes of the cross-domain environmental governance model. From the perspective of the deterritorialization process that environmental pollution is undergoing from local to wide-area, cross-regional environmental collaborative governance is a dynamic evolution process of connection and interaction between multiple jurisdictions. Existing research has constructed a theoretical framework for cross-domain environmental collaborative governance from multi-level and multi-dimensional perspectives, and conducted narrative representations and dynamic correlation studies to answer the question of what collaboration should be like and how to forge a good collaborative order. However, as far as collaboration is concerned, there is still much to explore about the role of dynamic mechanisms in shaping collaborative outcomes during the process. Based on the environmental collaborative governance of the Yangtze River Delta Integrated Demonstration Zone, a theoretical schema of “idea-system-practice” was constructed, and the key to unlocking the source of motivation for cross-domain ecological cooperation was explored from three dimensions: triggering mechanism, driving mechanism and action mechanism. The study found that the collaborative model of the Yangtze River Delta Integrated Demonstration Zone in environmental governance is a typical combined collaboration, with a tendency to promote green collaboration through system collaboration and the inherent attributes of an unchanged system change mechanism. This model is a rational selection of the Yangtze River Delta Integrated Demonstration Zone under the dual effects of incentive constraints and functional division. “Combined synergy” can obtain effective legitimacy and constitutional feedback in dealing with cross-border environmental pollution issues in the Yangtze River Delta Integrated Demonstration Zone, and achieve sustained synergy effects because it adheres to the logic of symbiosis, co-progress, and sharing.